1. Field of the Invention
The present invention relates to aqueous based fluids having polymer viscosifiers used as wellbore treatment fluids, and to methods for using such fluids in operations such as drilling, completion, and well workover operations in subterranean formations. More particularly, this invention relates to compositions and methods for improving or extending the performance of such aqueous based, polymer containing, drilling fluids at high temperatures.
2. Description of Relevant Art
Most drilling, completion, and workover fluids used in wellbore operations in subterranean formations, particularly in the search for oil or gas, are aqueous based fluids. Such fluids commonly achieve certain desired Theological properties such as shear thinning viscosity from addition of clays such as bentonite or from addition of polymers such as polyacrylamides, celluloses, cellulose derivatives, Scleroglucan polysaccharides, and Xanthan polysaccharides. Although each type of fluid has advantages and disadvantages, aqueous based fluids with polymer viscosifiers are becoming increasingly preferred over aqueous based fluids with bentonite or other clay based viscosifiers.
In drilling operations, it is conventional for known aqueous, polymer compositions to include water soluble salts for various purposes including shale inhibition, hydrate suppression, osmotic balance between the fluid and the formation, and solids free fluid weighting agents. Such salts are typically halide salts (i.e., chlorides and bromides) of mono-or divalent cations, such as sodium, potassium, calcium and zinc. Examples of such halide salts include without limitation sodium chloride, potassium chloride, calcium bromide and zinc bromide. Formate salts have been found to enhance the thermal stability of aqueous, polymer solutions.
Extended thermal stability of drilling, completion and workover fluids is a common problem in drilling and well operations. Temperatures in subterranean formations typically rise about 1° C. per hundred feet (30 meters) depth. Various aqueous, polymer compositions have their own characteristic temperature above which they undergo severe chemical degradation with accompanying undesirable reduction of viscosity, which limits their use in drilling and well operations below a corresponding depth. A thermally stabilized fluid is typically needed for wellbore operations at temperatures above about 250° F. As the oil and gas industry continues to drill to deeper depths in and to encounter higher temperatures, there continues to be an increasing interest in and need for fluids that will be stable at such depths.